That simmering debate among fliers escalated Sunday aboard a United Airlines flight that was diverted after two passengers argued over the use of a device that blocks reclining.

A Knee Defender, a gadget that “helps you stop reclining seats on airplanes so your knees won’t have to,” was at the root of the disturbance.

United Flight 1462 from Newark to Denver was forced to divert to Chicago’s O’Hare airport, according to United Airlines. The airline said that it does not allow the use of the recline-blocking device on its aircraft.

A federal law enforcement source, speaking on background, said the incident involved a male and a female passenger, both 48 years old.

The female passenger was unable to recline her seat and flagged a flight attendant.

The flight attendant told the man seated behind her to remove the Knee Defender device, but he declined. The female passenger then threw water in his face. The pilot decided to divert the flight to Chicago, where authorities met the aircraft.

The two passengers were removed from the plane before it continued on to Denver, United said. The passengers were not arrested.

The argument occurred in the Economy Plus section, which provides United passengers up to 5 inches of extra legroom compared with standard coach seats.